This invention relates to high pressure electric discharge devices, such as mercury or metal halide arc lamps, and particularly to the means for gettering hydrogen in such lamps.
High pressure discharge devices generally comprise a fused silica arc tube containing a fill of mercury or mercury and metal halides and which is supported by a wire frame within an outer bulbous envelope containing an inert gas such as nitrogen. It is well known that hydrogen contamination is detrimental to the operating of such devices. When trapped in the bulbous envelope, the hydrogen diffuses through the fused silica wall of the arc tube and adversely affects both starting and reignition voltages. The hydrogen migrates into the arc tube and forms, in the case of iodine fills, hydrogen iodide, which is a volatile iodine-containing species and exists as a gas at temperatures even as low as -20.degree.F. At low ambient temperatures the effect of hydrogen contamination is especially noticeable because the presence of the corresponding iodide produces high starting voltages. Moreover, the presence of hydrogen iodide in the arc tube results in a high value of voltages required to reignite the lamp each half cycle of alternating current during the warm-up phase of the lamp operation. This voltage, referred to hereinafter as "reignition voltage," is an important parameter in determining whether a lamp can operate reliably on a given ballast circuit. The lower it is, the more reliable will be operation, or conversely, the more economical will be the ballast design to reach a desired level of reliability.
One of the sources of hydrogen in such devices is the bulbous glass envelope. Ultraviolet light emitted from the arc tube releases hydrogen from hydroxyl radicals which are entrapped in the glass outer envelope.
Getters, that is materials which entrap extraneous gases, have previously been utilized in such devices. Gettering, as usually practiced in the art, involves flashing or volatilizing barium metal to react with gases, thereby removing them from the system. However such procedures not only remove the hydrogen but also getter the nitrogen which is intentionally added. Since an inert gas should generally be present within the envelope, replacement of the nitrogen with argon would be required, as argon is not gettered by barium. But because the use of argon reduces the potential where arcing between elements of the lamp can occur, it is not as satisfactory as nitrogen. Thus, the use of conventional barium getters has serious disadvantages. The same is true of the so-called flashless getters, such as tantalum, cerium, or alloys containing these metals, such as are known to the art. All of these react rapidly with nitrogen as well as hydrogen and would require replacement of the nitrogen fill gas of the outer envelope by argon.
A method of removing hydrogen from the outer envelope, without appreciably effecting the nitrogen content, is described by U.S. Pat. No. 3,519,864, assigned to the assignee of the present application. This patent employs barium peroxide as the getter and disposes the material at a location in the outer envelope where the temperature is normally expected to lie between 150.degree. and 427.degree.C. Within this temperature range, barium peroxide effectively getters hydrogen without significantly reacting with the nitrogen fill gas. The gettering rate at a hydrogen pressure of 30 Torr for 0.7 grams of BaO.sub.2 is 25 millitorr-liter/minute at a temperature of 300.degree.C. However at this temperature, the reaction results in an oxygen equilibrium partial pressure of 0.2 millitorr. Such a partial pressure of oxygen is objectionable on several counts. First, nickel platted frame parts begin to show evidence of oxidation after about 100 hours of lamp operation, which could lead to weld failures and presents a generally unsightly appearance. Secondly, the molybdenum arc tube leads show the formation of white cyrstalline form, molybdenum trioxide after about 100 hours which could lead to failure of the hermetic molybdenum ribbon seal resulting in an arc tube leaker. Thirdly, the liberated oxygen may make the identification of outer jacket leakers difficult in manufacture.
In order to effectively getter hydrogen, while reducing the concentration of liberated oxygen and the deleterious affects associated therewith, a getter of barium peroxide and copper has been described for use with high pressure electric discharge devices in U.S. Pat. No. 3,737,710, assigned to the assignee of the present application. A significant disadvantage of the barium peroxide and barium peroxide-copper approaches, however, is that the getter package employed is quite expensive.
A much less expensive gettering means (as little as one tenth the cost of the barium peroxide packages) that has been employed in arc discharge lamps comprises the use of strips of zirconium or zirconium-aluminum, as described, for example, in U.S. Pat. Nos. 2,749,462 and 3,805,105, respectively. However, these zirconium and zirconium alloy getters do not have the high degree of selectivity and hydrogen solubility as the barium peroxide types. Further, these zirconium and zirconium-aluminum strips are very porous and, thus, tend to adsorb contaminants. Hence, oil contamination problems are not uncommon in such zirconium getter lamps.